Engine Performance at altitude~am I being taken for a ride?

keninaz

Chief Petty Officer
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Dec 15, 2010
Messages
448
I am looking at the purchase of a early 1970s 14 foot Boston Whaler with a 35 hp Evinrude motor.
But before I have even seen the boat I am being told that with 4 people in the 14 foot boat that the boat would not even make 10 mph on a lake at about 6500 feet. The owner it 1st blamed the carburetor and then said that he was told that you must use a different prop at altitude.
As I am not used to dealing with altitude problems but I am familiar that gas motors do lose power at altitude I am a bit confused by his statements. Is he correct about a different prop or are we dealing with a low-power output or possible carburetion problem?
I just cannot afford to buy a pile of junk with my retirement money.
 

Robbabob

Senior Chief Petty Officer
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Jun 5, 2009
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678
I have heard of this, but someone with the experience will chime with what you need to know.
 

Bob_VT

Moderator & Unofficial iBoats Historian
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26,019
Changing the prop is not the answer, Do an internet search on High Altitude Outboard Jetting and you will get a better understanding.
 

keninaz

Chief Petty Officer
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Dec 15, 2010
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448
Yes I am familiar with jetting at high altitudes for cars anyway. Back I the day I did many on cars.
But for a boat to lose all that performance with 4 on board with a 35 HP motor bothered me and it sounded more than jetting could correct?
 

Franzel311

Seaman
Joined
Sep 17, 2014
Messages
62
Could be weak compression in a cylinder , it could be hard to notice with one or two ppl in the boat but he had four on board at that alt the boat had no power and that's what he's blaming it on
 

keninaz

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Dec 15, 2010
Messages
448
That is what I am thinking and I don't need to buy something that needs work right away.
 

Franzel311

Seaman
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Sep 17, 2014
Messages
62
That's a easy check tho get a compression guage pop out the plugs one buy one check for transfer if u c any u know to bail right away if not run the check
 

UncleWillie

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Oct 18, 2011
Messages
3,995
From an Engineering perspective......

An Internal combustion engine is and Air Pump at its heart.
The Amount of Power you can produce is dependent on how much fuel you can burn.
The amount of fuel you can burn is dependant on how much Oxygen that is available.
Assuming a constant Temperature and Humidity, going from Sea level to 6500 feet, The the Air Density is ~25% less.

35hp * 75% = ~25hp

If it is colder and/or drier in the Mountains, the actual loss will be less.
If you are Not coming from True Sea Level, the apparent loss will be less.

Expect your 35hp engine to act like ~25-30hp. A bit less power but not a disaster.

With 4 people on board, it was likely at close to Max weight. That will matter!
 
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JimS123

Fleet Admiral
Joined
Jul 27, 2007
Messages
7,952
I am looking at the purchase of a early 1970s 14 foot Boston Whaler with a 35 hp Evinrude motor.
But before I have even seen the boat I am being told that with 4 people in the 14 foot boat that the boat would not even make 10 mph on a lake at about 6500 feet. The owner it 1st blamed the carburetor and then said that he was told that you must use a different prop at altitude.
As I am not used to dealing with altitude problems but I am familiar that gas motors do lose power at altitude I am a bit confused by his statements. Is he correct about a different prop or are we dealing with a low-power output or possible carburetion problem?
I just cannot afford to buy a pile of junk with my retirement money.
I have a boathouse full of old motors, so "old" isn't new to me. But then again, its a hobby and I like to tinker. If they don't run and I have to throw parts at them I figure that's what I saved all that retirement money for in the first place.

Whalers are fine boats and they hold their value, but IMHO the old ones are way over priced. If the motor is an early 1970's as well, you better budget on possibly spending a lot of money to make her reliable. If cost is an issue, you might be better off looking for a newer model.

Some early 70's outboards were problematic - that was one of the worst periods IMHO. By 1977-78 they improved significantly.
 

rickdb1boat

Supreme Mariner
Joined
Jan 23, 2002
Messages
11,195
I would go into the purchase with only the boat value in mind. An early 1970ish 35 HP engine will not be worth much anyway. If the boat is truly what you want, assume that you will need to repower it in the not to distant future. I think your overall boating pleasure would be greatly increased if you had a newer, more reliable outboard. An Evinrude Etec would be my choice for that boat. But I do realize that money can be an obstacle...
 

Frank Acampora

Supreme Mariner
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Jan 19, 2007
Messages
12,004
No, you are not being "Taken for a ride." YES YOU WILL NEED TO CHANGE THE PROP TO A LOWER PITCH. Because there is less oxygen at altitude, you must re-jet to a smaller size so the engine does not run rich. THEN, because the engine is not producing the same power as at sea level. you must reduce pitch so you don't load the engine down below the factory recommended WOT RPM range. And yes, even though the Whaler is a "flat bottom", with four people and an engine producing somewhat less than 35 HP, you simply will not get what the hull is capable of at sea level.
 
Last edited:
Joined
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....." YES YOU WILL NEED TO CHANGE THE PROP TO A LOWER PITCH. Because there is less oxygen at altitude, you must re-jet to a smaller size so the engine does not run rich. THEN, because the engine is not producing the same power as at sea level. you must reduce pitch so you don't load the engine down below the factory recommended WOT RPM range.......


Wow, it took 10 replies before someone pointed this out. :lol:
 

fishrdan

Admiral
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Jan 25, 2008
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6,989
I agree with Frank, the prop (lower pitch) change is a must.

For re-jetting the carb, are you always going to be running at high altitude or going back-forth between low-high elevation? I have re-jetted for high altitude when I was going to be up there for a period of time, also just ran the boat with sea level jetting on other occasions as I was only going to be up in altitude for 1 day.. If you run with sea level jetting at 6500', the outboard will run like carp. I used the OB to run across lake at WOT, then fished off the electric trolling motor.

As for the boat's poor performance at altitude, that's to be expected.

What is the hull's HP rating? Ideally, a high altitude boat will have the hull's max HP rating, to help overcome power loss at altitude and let the boat perform well.
 

Franzel311

Seaman
Joined
Sep 17, 2014
Messages
62
I used to race stand up jet skis professionally all over the country I could b racing in the Ocean one weekend and b at a late alt 5,000 the next weekend and yes I know this is like comparing apples to oranges but it is still a VERY touchy 2 stroke engine compared to this touchy old 2 stroke engine every race I went to if the elevation changed by more than 400ft I had to rejet and tune appropriately and I would achive my target RPM at WOT I NEVER HAD TO CHANGE the Pitch of my impeller the only time I would have to do that would b for cold salt water VS. warm fresh water as the latter was denser....and my experience doesn't just end there iv been running fast out board motor and I/O boats my hole life at all diff elivations if your motor is running and is TUNED properly you should not have to (regear persay) to achieve optimum RPM at WOT/ By regearing you r compasating for another issue...that's just my opion
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
If your engine has still good and even compression on all cylinders, it's a combination of less diam jets and less pitch prop to compensate better high altitude issues. Check if Evi could have high altitude jet kits, or information regarding high altutude iuse. Just don't expect a 100% engine performance as opposed to sea level use.

Happy Boating
 

Silvertip

Supreme Mariner
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Sep 22, 2003
Messages
28,758
A naturally aspirated eingine loses 2 - 3% of its power for every 1000 feet above sea level. Jetting changes account for the lack of oxygen at altitude so it allows the fuel/air mixture to be in the 14:1 ratio. Jetting does not bring back the lost power caused by the altitude change. Therefore it reasonable to expect to change the prop to a lower pitch to provide reasonable performance. But in the end, 6000 feet means the engine loses (worst case) 18% (6 x 3%) of its power. 18% of 35 HP is 6.3 HP. You therefore have a 28.7 HP engine at that altitude. That lack of knowledge about HP loss is why some flat land small airplane pilots sometimes end up in trouble when flying into high altitude airports. Compensation must be made. In the case of aircraft you can lighten the load.
 

Cap'nHandy

Seaman
Joined
Aug 27, 2014
Messages
64
What Silvertip said. ^^^ You will need to rejet and reprop for best performance.

50hp at altitude should be about like 35 at sea level from a practical standpoint.

Hey Silvertip - is that a Citabria? Bottom of wing looks too flat to be a Decathlon... But it is a tiny picture!

Blaine
 

racerone

Supreme Mariner
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Dec 28, 2013
Messages
36,033
Weigh the boat and check it for being " water logged " it does happen to some.
 
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